1. Field of the Invention
The present invention relates to an improved apparatus for the manufacture of magnetic disks carrying very thin magnetic layers by applying a fluid dispersion of magnetic materials in a binder mixture to rigid disk substrates by a spin-coating process while controlling the coating thickness and coating profile.
2. Description of the Prior Art
Magnetic recording media in the form of rotating disks are extensively used as memories which allow rapid access to any desired position on a disk. Two important factors relating to such disk memories are the amount of information which can be stored thereon, which is usually described as the recording density and is expressed in bits per inch, and their magnetic properties, which influence the output signals of the magnetic disk. The achievable recording density in turn depends on the thickness of the magnetic coating, and in general, the thinner the coating, the higher the recording density can be.
There is a growing demand for magnetic disks which produce higher and higher recording densities while still retaining an output signal which is suitable for further processing. If these very thin magnetic coatings are used, the heads, which fly on a layer of air which is present between the magnetic disk and the head, have to be brought close to the recording medium. In the case of magnetic disks which have a magnetic layer about 1.mu. thick, the distance between the flying head and the disk surface is so small (the distance being only from 0.2 to 0.6.mu.) that the recording surface must be extremely flat.
It has been known in the art for many years to manufacture disk memories by coating an aluminum disk substrate with a liquid dispersion which contains a magnetizable material finely dispersed in a curable binder, subsequently drying and/or curing this coating and, if desired, also buffing and/or polishing it. A coating technique which has proved particularly successful is the spin-coating process, in which the disk substrate is caused to rotate and the coating mixture is allowed to flow onto it, as is described for example in U.S. Pat. No. 3,198,657. To achieve a thin magnetic layer, the excess dispersion which has been applied is then spun off.
In the production of magnetic coatings for magnetic disks, a suitable method has proved to be to pour the dispersion which usually contains finely divided; magnetic pigments, (e.g. gamma-iron oxide, magnetite or ferromagnetic metal) with particle sizes of from about 0.2 to 1.mu., and binders, especially heat-curable binders, such as epoxy resins, phenolic resins, aminoplast precondensates, polyester resins, polyurethanes or polyurethane-forming compounds, or mixtures of such binders with one another or with other binders, such as polycarbonates or vinyl polymers, for example vinyl chloride or vinylidene chloride copolymers, or heat-curable acrylate or methacrylate copolymers. In general, the magnetic dispersions also contain volatile solvents to produce a dispersion which can be poured through a nozzle, for example, tetrahydrofuran, toluene, methyl ethyl ketone and the like, the type of solvent used depending, inter alia, on the binder employed.
After cleaning the base disk substrates with solvent, the magnetic dispersion is poured from a nozzle under slight pressure in the spin-coating process, wherein an excess of the magnetic dispersion is deposited on the rotating disk substrate from the nozzle which can be moved parallel to the disk substrate surface in a radial direction. During such pouring in one method, the nozzle moves from the inner edge to the outer edge of the disk or preferably from the outer edge to the inner edge and then from the inner edge back to the outer edge. In an alternate embodiment, the nozzle moves radially from the outside diameter toward the inside diameter of the substrate without pouring any dispersion, and pouring does not begin until the nozzle nears the inside diameter of the substrate. Disks produced by such a spin coating process usually have an inherent wedge in the coating thickness, this thickness increasing from the inside diameter (ID) through the middle diameter (MD) to the outside diameter (OD) of the coated disk as a result of the spin coating method.
As indicated above, there is a continual need for higher performance magnetic recording disks and such higher performance means thinner and thinner magnetic recording media. To date, the methods used to generate the thinner media has been to adjust the coating viscosity and solvent balance, spin the disk at increased RPM's or grind more coating off in the buffing operation. None of these techniques seem to be a viable approach for achieving the thinner disks of the future. Control of the coating wedge has been nearly impossible using the above-mentioned techniques. In the past, this has not been a major problem because the natural wedge of 10.mu." created during the spin coating operation was approximately what was needed for the finished disk. Future disk requirements may include a flat recording media or a near no-wedge disk.
It has been discovered that by placing a fluid-flow barrier over a magnetic oxide coated disk during the spin-off portion of the spin coat cycle, the barrier having an opening, groove, or tunnel therein, the coating thickness can be reduced by 30-50%. It has also been found that this innovation can be used to produce any coating wedge desired.